Kōshirō is a computer expert and spends most of his free time researching on his laptop (which resembles a Tangerine Apple iBook, only with a pineapple symbol). He is very curious and is always making theories and trying to understand the world around him. He starts out as a very introverted person, who isn't very in touch with other people and doesn't really understand their feelings. Being logical and thoughtful, he doesn't appreciate impulsiveness and gets angry when people around him act before thinking. He used to be on the soccer team with Takenouchi Sora and Yagami Taichi, though this was only to avoid contact with people as he isn't very good at playing soccer. He is the partner of Tentomon and the bearer of the Crest of Knowledge

Kōshirō's biological father was a genius mathematician and a lecturer at a university; he and Kōshirō's biological mother died in a car crash when he was a baby. He was adopted by Izumi Masami, a distant relative of his father's and his wife Kae, who had suffered the loss of their own baby boy not long before the accident. Kōshirō was unaware for most of his life, and once he found out, he was devastated. However, he had since learned to accept himself as a true Izumi.

In 1995, he lived in Hikarigaoka for a few months with his parents. However, he witnessed the fight between a Greymon and a Parrotmon that was later attributed to a terrorist bombing. Not wanting to stay in such a dangerous place, his family moved to Odaiba.

Sometime before Digimon Adventure begun, Kōshirō overheard his parents talking about whether they should tell him he is adopted or not. He took this hard, believing he is not a true Izumi and thus a disgrace, and started to subconsciously distance himself from people, especially his parents.

Kōshirō was the brains of the group, and he frequently helped the group out of difficult situations. He became the owner of the Crest of Knowledge, not only because he was extremely smart, but because he also had a hunger for the truth.

When Taichi and Metal Greymon disappeared following the defeat of Etemon, Kōshirō and Tentomon split from the group to look for Gennai, hoping he had answers for Taichi's disappearance and the children's role as Chosen Children. However, due to Pico Devimon's meddling, they ended up trapped in Vademon's pocket universe. Threatened by Vademon by being sent to hell, Kōshirō gave up his inquisitive heart to save them, and was stuck in an odd trance, rejecting Tentomon and his Crest. As Tentomon devolved into Bubbmon, Kōshirō realized what he had done and recovered his inquisitive heart and Crest with Bubbmon's help. His desire to become curious again ended up activating his Crest, and the newly-evolved Atlur Kabuterimon defeated Vademon and destroyed his pocket universe.

After finding Gennai, Kōshirō learned from him about attributes. This was key in activating Vamdemon'sDigital Gate, as the cards to the gate were sorted by levels and attributes.

At one point, when Kōshirō and his parents were hiding away during the siege on Odaiba, Kōshirō's parents decided to discuss the subject of his adoption, revealing to him that his parents died in a car crash when he was a baby and Kōshirō's adoptive father, a distant relative of Kōshirō's real father, adopted the orphan as his own. After learning this, Kōshirō accepted his past and truly accepted his adoptive parents as his own.

In Our War Game, Kōshirō was the one who first discovered a DigiTama infected with a computer virus on the Internet. He rushed off to alert the others, at first only successful in finding Taichi. He played a key role as the brains behind the attacks that were implemented by Taichi and Yamato.

Following his adventures in the Digital World, Kōshirō decided to leave the soccer club and join the school's computers club, where he would fit better. Due to his expertise, he eventually rose to become his new club's president in the sixth grade. According to Inoue Miyako, he was quite revered by the other members of the computers club.

In May 2000, Kōshirō arrived at the Digital World and gave up his Crest power to free the Four Holy Beasts. As a result, Tentomon lost the power to evolve to Perfect.

Although he grew, he was still short compared to the other Chosen Children. As he was only in his first year of middle school, Kōshirō had a lot of free time comparing to some of the other old Chosen, and often stopped by the Odaiba Elementary computer lab to help the new Chosen Children. He was still very much into computers, and was curious about the new items the new Chosen had access to.

Like in Our War Game, Kōshirō was responsible for the planning and execution of the attacks on Diablomon. He was the one who gathered all the Chosen Children (including Tachikawa Mimi who flew in from America) to help defeat Diablomon and stop the havoc it wreaked on the net and the real world.

By the year 2027, Kōshirō set up a research team to study more of the Digital World than what the human population already knows. He works with Takenouchi Haruhiko, Sora's father, and Kido Shuu, Jou's older brother. He is married and has a daughter, who has a Motimon as her partner.